Brief overview of composite slabs and columns

Composite slabs contain reinforced concrete cast on top of profiled steel decking that functions as formwork all throughout construction and external reinforcement at the final stage.

Composite slabs:

• Comprise of profiled steel decking with an in-situ reinforced concrete topping.
• The decking (profiled steel sheeting) not only functions as permanent formwork to the concrete, but also arranges adequate shear bond with the concrete in order that when the concrete attains strength, the two materials function compositely side by side.
• Spacing should remain among 3 m and 4.5 m onto supporting beams or walls.
• When the slab is unpropped at the time of construction, the decking alone can withstand the self weight of the wet concrete and construction loads. Succeeding loads are implemented to the composite section.
• When the slab is propped, all of the loads should be countered by the composite section.
• The slabs are generally designed as simply supported members in the normal condition

Profiled steel sheeting:

• Depths vary from 45 mm to over 200 mm
• Yield strengths vary from 235 N/mm2 to minimum 460 N/mm2
• 0.8 mm and 1.5 mm thick
• The different shapes offer Interlock among steel and concrete
• Decking may also be applied to make the beams stable against lateral torsional buckling all through construction.
• Make the building stable entirely by performing as a diaphragm to transmit wind loads to the walls and columns.
• Temporary construction load generally governs the choice of decking profile.

Composite Columns: A steel-concrete composite column stands for a compression member that includes either a concrete encased hot-rolled steel section or a concrete filled tubular section of hot-rolled steel. The existence of the concrete is permitted in two ways.

• Protection from fire
• It is assumed to resist a small axial load
• Composite columns minimize the effective slenderness of the steel member that improves its resistance against axial load.

The bending stiffness of steel columns with H-or I-section is much higher in the plane of the web (‘major-axis bending’) than in a plane equivalent to the flanges (‘minor-axis bending’). The ductility performance of circular type of columns is considerably superior as compared to rectangular types. It is not necessary to arrange additional reinforcing steel for composite concrete filled tubular sections. Corrosion protection is arranged with concrete to steel sections in encased columns.

When local buckling of the steel sections may be detached, the cutback in the compression resistance of the composite column because of overall buckling should be permissible positively. The plastic compression resistance of a composite cross-section demonstrates the maximum load that can be implemented to a short composite column.



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Published By
Rajib Dey
www.constructioncost.co
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On site demonstration of mixing and placing concrete

In this construction video tutorial you can get live demonstration of blending and positioning of concrete in the jobsite for slab or road.

Mixing: Mixing of concrete is one of the most crucial factors for obtaining good final properties and for this purpose proper equipment should be utilized.

So, it is suggested to employ ready-mixed concrete to get the best result. Mixing allocates the aggregate uniformly during the cement paste and it makes sure that all of the cement is completely saturated in water, and eliminates large air voids.

Besides, mixing separates agglomerated clusters of cement particles and facilitates air entraining admixtures to produce the perfect air void system. Undermixing can produce large flaws and therefore leads to poor strength, whereas overmixing consumes time and energy and can demolish entrained air voids. If the workability is lower, the more mixing energy and mixing time is necessary.

Placing: As soon as the concrete is blended sufficiently, it should be placed into the formwork to determine its final position and shape. If it is required to reinforce the concrete, the rebar should have been placed properly in order that the concrete can flow around it.

If the concrete mixing truck is available adjacent to the site, then the concrete should be directly poured into the forms. If it becomes difficult, the concrete is transmitted in buckets through a crane or wheelbarrow.

If it is impossible because of the necessary distance or the size of the job, the fresh concrete has to be pumped via a system of pipes or hoses to the site with the special concrete pumps. Concrete that is to be pumped contains more severe needs for workability.

If the concrete gets too dry, it will not pump properly, whereas if it gets too wet it will have a tendency to segregate. Segregation may also happen if the concrete is poured into the formwork too rapidly, because larger aggregate particles will be subject to be pushed downward.

In slab construction, the placing has to be initiated along the perimeter at one end of the work with each batch arranged against transmitted concrete earlier. It is suggested not to dump the concrete in independent piles and then level and work them jointly; nor should it be placed in large piles and carried horizontally into final position.

Watch the following video for more details.

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Published By

Rajib Dey

www.constructioncost.co

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